Glass is, in many respects, an ideal cladding material for buildings. It has an aesthetically pleasing look that is extremely durable compared to other materials, and it is maintenance free except for occasional cleaning. In its natural state, it is clear and may be tinted or coated to control appearance. It may be made fully transparent to provide a view and admit direct sunlight, or it may be made translucent or opaque via etching or coating. In the latter case it will admit diffuse light, which provides a far superior quality of natural light and helps avoid glare and localized overheating characteristic of direct beam sunlight.
The most common form for glass as building material is in flat sheets, produced by the float process. Such flat glass is either used in its monolithic form, or fabricated into “insulating glass units” characterized by two or more glass panes, known as lites, each lite being separated by a spacer around the perimeter. The most common range of thicknesses for lites of glass is 3 mm to 6 mm (⅛″ to ¼″). Typically, the airspace in an insulating glass unit is on the order of 12.5 mm (0.5″). The spacer does not provide structural rigidity and such glass units have to be attached to the building by a framing system that extends around the glass unit.
Despite all its good qualities, flat glass can be challenging to use in building situations because it is relatively brittle and low in strength. It can be easily broken by application of stress. As a result, in typical applications, glass must be supported around its entire perimeter by a framing system. The framing system must support the glass uniformly, such that any force applied to the glass in reaction to wind load (or, in the case of sloped glass, dead load) is distributed as possible over the perimeter. The edge of the glass must be clamped in a manner that is free from angular constraint around an axis parallel with the perimeter in order to prevent stress concentration.
These stringent requirements are generally met by the use of window framing and curtainwall framing. These framing systems hold the glass at the perimeter without angular constraint of edges, either by clamping the glass between elastomer seals, or by use of a structural elastomer adhesive, typically silicone. The framing system, which is fixed to the building, must be made from linear elements that are straight and true, and these elements must be assembled so that they are in a common plane, in order that the supporting surface for the glass be flat at the time of installation. The linear elements that make up the framing system must also be substantial (that is, have sufficient moment of interia), in order to remain flat under load (typical specification for maximum deflection under windload is length/175). Therefore, the framing system must be carefully manufactured from elements that have significant structural value, especially in larger-sized window and glazing systems.
Although the use of flat glass in window and curtainwall systems is commonplace, highly evolved and reliable, the need for framing and specialized glazing techniques contributes greatly to the price. It is not uncommon for the cost of the glass to represent 25% or less of the installed cost of the cladding system. The other 75% or more of the installed cost is for framing and installation cost; or in other words, framing and installation can represent more than three times the cost of the glass itself. As a result, the cost per unit area to clad openings or sections of buildings with conventional glass systems can greatly exceed the cost per unit area to clad the same opening with opaque claddings, which by their nature are not subject to the stringent stress management requirements that apply to glass. Often the price differential between conventional glass claddings and opaque claddings is two times or more.
Cost premiums that result from framing requirements imposed by the lack of inherent structural strength influences the entire field of architecture and construction. Budget considerations often forces building designers to use opaque materials where glass may have been desirable. This may occur either at design stage or during rounds of ‘value engineering’ necessary to trim costs when building designs exceed budgets. This is particularly relevant in buildings where lowest capital cost is a dominant criterion, such as industrial buildings or publicly funded schools. As a result, many building occupants do not receive the benefits of view and natural light that can be obtained through the appropriate use of glass in building designs.
Frameless ‘point-supported’ glass systems are available in today's marketplace. They hold glass via metal attachments called spiders, which are either fixed through holes drilled through the corners of the glass, or by high-performance adhesives. These systems rely on the glass itself to provide the rigidity necessary to work with point support systems. The goal of these systems is usually to achieve an elegant, highly transparent aesthetic, and they are not intended as a cost-effective clad over structure system. Because point-support systems do not support glass around the perimeter, they require increased glass thickness, compared to the glass thickness required by window and curtainwall systems which support the glass around the perimeter. Such “thick” glass typically has a thickness of 9 mm or more.
There are numerous opaque panel systems in use worldwide in the construction industry for building cladding. Common panels include metal-clad foam, metal-clad honeycomb, concrete, and stone. Opaque panels are designed to have sufficient structural strength to resist windload and other loads that may be applied to them. Depending on the system, panels are attached to buildings by a number of methods, such as framing similar to that used for glass systems (many panels can be glazed directly into curtainwall frames), or various clip systems including hook and pin.
There are a number of light-admitting plastic panel systems. For example, CPI daylighting (www.cpidaylighting.com) uses multi-wall polycarbonate sheets that have inherent structural capacity sufficient to bear wind load and dead load over the scale of a single panel. The material is relatively low modulus, and therefore sheets have sufficient flexibility to avoid stress concentration when clipped to structural members. Sheets may be semi-transparent, translucent, or opaque. Internal structure precludes total transparency. Kalwall (www.kalwall.com) is translucent panel system, based on panels comprising two sheets of thin (1.5 mm) fibre reinforced plastic, bonded to an aluminum 1 beam lattice structure of approximately 2.5″ thickness and in plane lattice dimensions of approximately 30 cm (1′)×60 cm (2′). Kalwall panels are held in place by framing and inter-panel clamps.